The present invention generally relates to semiconductor devices and in particular to a semiconductor device constructed on an epitaxial layer of a first semiconductor material grown heteroepitaxially on a semiconductor substrate or wafer of a second semiconductor material.
The compound semiconductor material of the group III element and the group V element such as gallium arsenide (GaAs) is a promising material for the substrate of the next generation semiconductor devices because of the various advantageous properties pertinent to the material such as the high electron mobility, characteristic band structure which facilitates the emission and absorption of photons by the direct transition process and the like.
In order to mass produce the compound semiconductor devices cheaply, the technique to produce a large diameter substrate is essential. However, such a process is not established yet partly because of the difficulty of growing a large diameter bulk crystal of compound semiconductor materials with a satisfactory quality in respect to the composition, dislocation density and the uniformity of the crystal and partly because of the difficulty in handling the brittle wafers sliced from the bulk crystal ingot. Because of the foregoing reasons, an alternative technique to obtain a substrate of the compound semiconductor devices by growing a thin layer of the compound semiconductor material heteroepitaxially on a silicon wafer is studied. It should be noted that the fabrication technique and the handing process of a large diameter silicon wafer is well established.
In the latter approach, however, there arises a problem in that the epitaxial layer of the compound semiconductor layer grown on the silicon wafer contains a substantial amount of dislocations because of the difference in the lattice constant and the coefficient of thermal expansion between the compound semiconductor material and silicon. It should be noted that when such a difference exists in the lattice constant and thermal expansion, there appears a slip or misfit in the crystal lattices of silicon and the compound semiconductor material at the heterojunction interface, and such a misfit is propagated into the epitaxial layer as dislocations.
In order to reduce the dislocations in the compound semiconductor substrate, various approaches are proposed such as annealing the epitaxial layer together with the silicon wafer so as to relax the stresses developed at the heterojunction interface, interposing a strained super lattice layer between the silicon wafer and the epitaxial layer for intercepting the dislocations and the like. However, any of these approaches is so far unsuccessful in reducing the dislocation density at the top surface of the epitaxial layer which is used as the substrate of the compound semiconductor device, to the level below about 10.sup.7 cm.sup.-2. This level of the dislocation density is unsatisfactory for the material to be used as the substrate of the semiconductor devices.